Redefining the Age of Clovis: Implications for the
Peopling of the Americas
Michael R. Waters, et al.
Science 315, 1122 (2007);
DOI: 10.1126/science.1137166
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REPORTS
1122
it is difficult to ensure that the distance between
antennas can be made large enough for this requirement to be satisfied. This difficulty is typically encountered when antennas are placed in a
laptop and the telecommunication wavelengths
are on the centimeter scale (e.g., Bluetooth or
Wi-Fi). An illustration of the benefit of timereversal subwavelength focusing to overcome
this difficulty is given in Fig. 3. A three-antenna
TRM is used to transmit a color picture to a
three-antenna receiving array. The original
picture is encoded onto three RGB (red-greenblue) color channels. Each corresponding figure
is represented by a bit series giving the gray
levels of each pixel on that particular channel.
Then the simplest modulation is used (a positive
pulse for bit 1, a negative one for bit 0) to create
three bitstreams with a data rate of 50 Mbit/s
each. The intended global data rate is thus 150
Mbit/s. Time reversal is used to focus each bitstream onto one of the antennas (one antenna for
each color) of the receiving array. Then the three
bitstreams are decoded and mixed to reconstruct
the color image. The communication is performed with two kinds of receiving arrays. The
first is “classical”; it consists of three dipolar
antennas with a l/30 spacing. The second is a
microstructured antenna array analogous to the
one previously described (Fig. 1). It turns out that
the image reconstructed with the classical array is
gray-scaled: Its colors are lost. Indeed, subwavelength spaced antennas are strongly coupled, that
is, they essentially receive the same signal.
Hence, each transmitted pixel is gray because
the three different antennas corresponding to the
three different color channels receive the same
gray levels. However, when the microstructured
receiving array is used, each color stream focuses
independently at each antenna. Consequently, the
relative weights of the RGB components of each
pixel are preserved and the image is transmitted
without major losses. This experiment shows that
our approach allows one to increase the information transfer rate to a given volume of space.
References and Notes
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788 (2004).
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(2005).
11. D. O. S. Melville, R. J. Blaikie, Opt. Exp. 13, 2127 (2005).
12. Z. Jacob, L. V. Alekseyev, E. Narimanov, Opt. Exp. 14,
8247 (2006).
13. T. Taubner et al., Science 313, 1595 (2006).
14. M. Fink, Phys. Today 50, 34 (1997).
15. C. Draeger, M. Fink, Phys. Rev. Lett. 79, 407 (1997).
16. B. E. Henty, D. D. Stancil, Phys. Rev. Lett. 93, 243904 (2004).
17. G. Lerosey, J. de Rosny, A. Tourin, A. Derode, M. Fink,
Phys. Rev. Lett. 92, 193904 (2004).
18. G. Lerosey, J. de Rosny, A. Tourin, A. Derode, M. Fink,
App. Phys. Lett. 88, 154101 (2006).
19. R. Carminati, J. J. Saenz, J.-J. Greffet, M. NietoVesperinas, Phys. Rev. A 62, 012712 (2000).
20. D. Cassereau, M. Fink, IEEE Trans. Ultrason. Ferroelectr.
Freq. Control 39, 579 (1992).
21. J. de Rosny, M. Fink, Phys. Rev. Lett. 89, 124301 (2002).
22. Further details are available as supporting material on
Science Online.
23. A. L. Moustakas, H. U. Baranger, L. Balents, A. M. Sengupta,
S. H. Simon, Science 287, 287 (2000).
24. This work was partially funded by the Agence Nationale
de la Recherche under grant ANR-05-BLAN-0054-01.
Supporting Online Material
www.sciencemag.org/cgi/content/full/315/5815/1120/DC1
SOM Text
Fig. S1
References
7 September 2006; accepted 19 December 2006
10.1126/science.1134824
Redefining the Age of Clovis:
Implications for the Peopling
of the Americas
Michael R. Waters1* and Thomas W. Stafford Jr.2
The Clovis complex is considered to be the oldest unequivocal evidence of humans in the Americas,
dating between 11,500 and 10,900 radiocarbon years before the present (14C yr B.P.). Adjusted 14C
dates and a reevaluation of the existing Clovis date record revise the Clovis time range to 11,050 to
10,800 14C yr B.P. In as few as 200 calendar years, Clovis technology originated and spread throughout
North America. The revised age range for Clovis overlaps non-Clovis sites in North and South America.
This and other evidence imply that humans already lived in the Americas before Clovis.
or nearly 50 years, it has been generally
thought that small bands of humans carrying a generalized Upper Paleolithic tool kit
entered the Americas around 11,500 radiocarbon
years before the present (14C yr B.P.) and that
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these first immigrants traveled southward through
the ice-free corridor separating the Laurentide
and Cordilleran Ice Sheets (1). These people
developed the distinctive lithic, bone, and ivory
tools of Clovis (2, 3) and then quickly populated
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ceiving array. A TRM made of eight commercial dipolar antennas is placed in the far field,
10 wavelengths away from the receiving array.
(The electronic part of the setup is described in
fig. S1.) When antenna 3 sends a short pulse
(10 ns), the eight signals received at the TRM
are much longer than the initial pulse because of
strong reverberation in the chamber (typically
500 ns). An example of the signal received at
one of the antennas of the TRM is shown in Fig.
2A. When antenna 4 is used as a source, the
signal received at the same antenna in the TRM
(shown in Fig. 2B) is considerably different,
although sources 3 and 4 are l/30 apart. When
these signals are time-reversed and transmitted
back, the resulting waves converge respectively
to antennas 3 and 4, where they recreate pulses
as short as the initial ones (Fig. 2, C and D).
Measuring the signal received at the other
antennas of the receiving array gives access to
the spatial focusing around antennas 3 and 4
(Fig. 2E). The two antennas can now be
addressed independently, because the focusing
spots created around them are much smaller than
the wavelength (typically l/30). The diffraction
limit is overcome, although the focusing points
are in the far field of the TRM.
The origin of the diffraction limit, and the
way to overcome it, can be revisited by using the
time-reversal concept and the Green’s function
formalism, without the explicit use of the evanescent wave concept (20–22). The time-reversed
wave, generated by a closed TRM, which converges to its source, is always followed by a
spatially diverging wave due to energy flux
conservation. Because the focal spot results from
the interference of these two waves, the timereversed field can always be expressed (for a
monochromatic wave) as the imaginary part of
the Green’s function (22). In a homogeneous
medium, the imaginary part of the Green’s
function oscillates typically on a wavelength
scale. To create focal spots much smaller than
the wavelength, one introduces subwavelength
scatterers in the near field of the source. Therefore, the spatial dependence of the imaginary part
of the Green’s function is modified to oscillate on
scales much smaller than the wavelength.
A promising application of time-reversal
subwavelength focusing is telecommunications.
One way that has been proposed to increase the
data rate of a communication system is to use
multiantenna arrays at both transmitter and receiver (23); different bitstreams sent from each
antenna of the transmitting array can be decoded
at the receiving array under the condition that the
medium creates sufficient scattering. It is also
generally stated that the spacing between the
receiving antennas must be larger than l/2 (23).
If these two conditions are fulfilled, the global
maximum error-free data rate, or “Shannon
Capacity,” is at best multiplied by the number
of transmitting antennas. Such methods are
referred to as MIMO (multiple input–multiple
output). However, from a practical perspective,
the contiguous United States. Clovis humans and
their descendants then rapidly populated Central
America and reached southernmost South America by 10,500 14C yr B.P. (1).
Identifying when the Clovis complex first
appeared and knowing the complex’s duration is
critical to explaining the origin of Clovis, evaluating the Clovis-first model of colonization of
the Americas, determining the role of humans in
the extinction of late Pleistocene megafauna,
and assessing whether people inhabited the
Americas before Clovis. We determined a more
accurate time span for Clovis by analyzing the
revised existing Clovis 14C date record and
reporting high-precision accelerator mass spectrometry (AMS) 14C ages from previously dated
Clovis sites. Our AMS 14C dates are on
culturally specific organic matter—bone, ivory,
and seeds—that accelerator mass spectrometers
can date accurately (4, 5) to precisions of ±30
years at 11,000 14C yr B.P.
Clovis technology has strong Old World antecedents, but Clovis-specific traits (e.g., fluted
lanceolate projectile points) probably originated
in the New World, south of the continental ice
sheets (3). Clovis tools and debitage identify
and unify archaeological sites over a broad geographic range. Clovis sites and artifacts cluster
in North America, especially in the contiguous
United States (1). A small number of Clovis
artifacts have been recovered from Mexico and
possibly as far south as Venezuela (6). Even
though Clovis covers a broad geographic range,
only 22 Clovis sites in North America have been
directly 14C-dated (Fig. 1, Table 1, and table S1).
The 14C dates from these sites traditionally place
Clovis between 11,500 and 10,900 14C yr B.P.
(1, 7, 8). However, the 14C dates from 11 of these
sites are problematic and do not provide accurate
or precise chronological information to determine
the age of Clovis (5).
Three sites (East Wenatchee, Washington;
Blackwater Draw, New Mexico; and Cactus
Hill, Virginia) have Clovis diagnostic artifacts
but lack precise ages (5). Three sites (Lubbock
Lake, Texas; Kanorado, Kansas; and Indian
Creek, Montana) fall within the Clovis age
range but lack diagnostic Clovis artifacts (5).
The site of Sheriden Cave, Ohio, provides only
bracketing ages for Clovis artifacts (5). Questions exist about the accuracy of the 14C dates
from Aubrey, Texas (5), where diagnostic Clovis
artifacts were found. We obtained three dates
from the Sheaman site, Wyoming, that averaged 10,305 ± 15 14C yr B.P. These dates indicate that the Clovis context at Sheaman is
mixed with younger cultural materials (5). Finally, associations between Clovis artifacts and
14
C-dated faunal remains at two sites (Wally’s
Beach, Canada; and Union Pacific, Colorado)
are unresolved (5). Because of these problems,
we excluded the dates from these sites in assessing the age of Clovis.
This leaves 11 sites with a total of 43 14C
dates (Table 1 and table S1) (5). These sites
have assemblages of Clovis artifacts in secure
geological contexts. Existing ages from five
sites (Anzick, Montana; Paleo Crossing, Ohio;
Lehner, Arizona; Murray Springs, Arizona; and
Jake Bluff, Oklahoma) already have highprecision 14C dates on credible materials. We
obtained nine new ages from seeds and highly
purified bone and ivory collagen for five
imprecisely dated sites (Lange-Ferguson, South
Dakota; Dent, Colorado; Domebo, Oklahoma;
Shawnee-Minisink, Pennsylvania; and Colby,
Wyoming) (4, 5). In addition, we obtained five
ages on human remains from the Anzick site,
Montana (5). We attempted to date samples
from Sloth Hole, Florida, but the samples
contained no collagen.
These 43 14C dates place the beginning of
Clovis at ~11,050 14C yr B.P. (reducing former
estimates by 450 14C years) and its end at
~10,800 14C yr B.P. (younger than previous
estimates by 100 14C years). Accurate calendar
correlation of 14C ages from the Clovis time
period is not currently possible because of
correlation uncertainties (9). The Clovis-period
segment of the INTCAL04 calibration is based
on 14C-dated marine foraminifera and is not
accurate for the Clovis time period (10). The
most accurate calibration for this time period is
provided by a floating European tree-ring
chronology that is provisionally anchored to
INTCAL04 (11). Using this tentative calibration
(11), we estimated that Clovis has a maximum
possible date range of 13,250 to 12,800 calendar
yr B.P.—a span of 450 calendar years (Fig. 2).
By taking the youngest possible calibrated age
for the oldest Clovis site and the oldest possible
calibrated age for the youngest Clovis site, a
minimum range for Clovis is calculated as
13,125 to 12,925 calendar yr B.P.—a span of
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REPORTS
1
Departments of Anthropology and Geography, Center for
the Study of the First Americans, Texas A&M University,
4352 TAMU, College Station, TX 77843-4352, USA.
2
Stafford Research Laboratories, 200 Acadia Avenue,
Lafayette, CO 80026, USA.
*To whom correspondence should be addressed. E-mail:
mwaters@tamu.edu
Fig. 1. Map showing the location of Clovis and other early sites. The numbers correspond to those
found in Table 1. Other sites are 31, Monte Verde, Chile; 32, Nenana Complex sites, Alaska; and
33, Broken Mammoth, Alaska.
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200 calendar years. The ages for all Clovis sites
overlap within this 200-year period, and this
time span probably represents the true range of
Clovis. However, the absolute calendar placement of the floating tree-ring record is disputed
(12). By an alternative calibration (12), the
maximum time range for Clovis is 13,110 to
12,660 calendar yr B.P., and the minimum time
range is 12,920 to 12,760 calendar yr B.P. (Fig. 3).
Regardless of the exact calendar dates, the 200year duration for Clovis remains secure because
the floating dendrochronological sequence provides calendar-year separations between two
14
C-dated sites.
The oldest Clovis sites (n = 3 sites) are
located in Montana, South Dakota, and Florida;
younger Clovis sites are located in the interior
(n = 5) of the United States and in the Southwest (n = 2) and East (n = 1). The distribution
of dated sites shows no clear indication of northsouth or east-west age differences that would
indicate movement of people in one direction or
another. Instead, Clovis technology seems to
have appeared synchronously across the United
States at ~11,050 14C yr B.P. This pattern of 14C
dates is compatible with two contrasting
hypotheses.
First, this pattern could support the idea that
there was a rapid spread of Clovis people across
an empty continent. Demographic models suggest that people exiting the ice-free corridor
could have occupied the contiguous United
States within 100 years or less (13). Although
there is much speculation about a coastal migration of the first Americans from both Asia
and Europe (14, 15), the revised date range for
Clovis reopens the possibility of a Late Glacial
migration through the ice-free corridor that separated the Laurentide and Cordilleran Ice Sheets.
People could have easily traveled through the
ice-free corridor after ~11,500 14C yr B.P. (1)—
at least 200 calendar years before the oldest
known Clovis date. The biface and blade industry of Nenana (16) was well established at
the Broken Mammoth site, Alaska, to 11,770 ±
210 14C yr B.P. (WSU-4351)—at least 300
calendar years before our oldest recalibrated
Clovis date. The Nenana lithic assemblage
shows strong similarities to the Clovis lithic
assemblage (17). It is possible that either
Nenana people or others with a biface and blade
industry traveled through the corridor, and once
south of the ice sheets, they developed the
technological hallmarks characteristic of Clovis
and spread rapidly across the continent.
An alternative interpretation is that the instantaneous appearance of Clovis across North
America represents the rapid spread of Clovis
technology through a preexisting but culturally
and genetically undefined human population in
North America (18). In this case, Clovis
technology could have been introduced to this
population through a Late Glacial migration of
Clovis or Clovis progenitors or developed in
situ from a pre-Clovis technology already in the
1124
Americas. Regardless of which hypothesis is
correct, our revised chronology indicates that
Clovis technology spread rapidly.
Faunal remains associated with dated Clovis
sites constrain the timing of the extinction of
Proboscideans at the end of the Pleistocene.
Mammoths and mastodons were an important
source of food and raw materials used to
manufacture bone and ivory tools (3), as well
as perishable items from soft tissues. Proboscidean remains are associated with seven of the
well-dated Clovis sites (Lange-Ferguson, Sloth
Hole, Dent, Domebo, Lehner, Murray Springs,
and Colby), and the last occurrence of mammoth in the United States is dated at ~10,900
14
C yr B.P. After this time, Clovis and sites of
other complexes (e.g., Goshen and Folsom)
contained only bison and other extant species.
The extinction of mammoth and mastodon
coincides with the main florescence of Clovis.
Our revised ages for Clovis overlap dates
from a number of North American sites that are
technologically or culturally not Clovis. The
earliest dated sites of the Goshen complex (Mill
Iron, Montana; and Hell Gap, Wyoming) (19)
overlap the age range of Clovis (Figs. 2 and 3,
and Table 1, and table S1). This indicates that
Goshen is either coeval with the entire range of
Clovis or briefly overlaps the later stages of
the Clovis time period. Clovis also overlaps
the date for the Arlington Springs human skeleton from Santa Rosa Island, California (Figs.
2 and 3 and Table 1) (20). No artifacts were
found with the Arlington Springs human remains, and his cultural affiliation is unknown.
The presence of human remains on Santa Rosa
Table 1. Summary of 14C dates from Clovis and Clovis-age sites. Single 14C dates, date ranges, and
averaged dates are reported. If multiple 14C dates were available from a single-component site, the
dates were averaged with the method in (28). All dates are given at 1s SD. n, number of dates.
Site
Date
(14C yr B.P.)
Clovis sites (credible ages and Clovis diagnostics)
1. Lange-Ferguson, SD (n = 3)
2. Sloth Hole, FL (n = 1)
3. Anzick, MT (foreshaft ages) (n = 2)
4. Dent, CO (n = 3)
5. Paleo Crossing, OH (n = 3)
6. Domebo, OK (n = 1)
7. Lehner, AZ (n = 12)
8. Shawnee-Minisink, PA (n = 5)
9. Murray Springs, AZ (n = 8)
10. Colby, WY (n = 2)
11. Jake Bluff, OK (n = 3)
Clovis sites (indirectly dated and Clovis diagnostics)
12. East Wenatchee, WA (n = 1)
Clovis-age sites (credible ages but no Clovis diagnostics)
13. Indian Creek, MT (n = 1)
14. Lubbock Lake, TX (n = 2)
15. Bonneville Estates, NV (n = 1)
16. Kanorado, KS (n = 2)
17. Arlington Springs, CA (n = 1)
Problematic Clovis and Clovis-age sites
18. Sheriden Cave, OH (above artifacts, n = 5)
Sheriden Cave, OH (below artifacts, n = 2)
19. Blackwater Draw, NM (n = 3)
20. Cactus Hill, VA (n = 1)
21. Wally's Beach, Canada (n = 4)
22. Union Pacific, WY (n = 1)
23. Aubrey, TX (n = 2)
24. Sheaman, WY (n = 3)
Ages from other early sites
25. Mill Iron, MT (Goshen) (n = 4)
26. Hell Gap, WY (Goshen) (n = 1)
27. Cerro Tres Tetas, Argentina
(pre-Fishtail, n = 5)
28. Cuevas Casa del Minero, Argentina
(pre-Fishtail, n = 2)
29. Piedra Museo, Argentina (pre-Fishtail, n = 2)
30. Fell’s Cave, Chile (Fishtail, n = 1)
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11,080
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11,040
10,990
10,980
10,960
10,950
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10,885
10,870
10,765
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